1. Field of the Invention
The present invention relates to a band pass filter apparatus, and more specifically to a filter apparatus including a ladder circuit.
2. Description of the Related Art
Ladder filters are widely used as band pass filters. For example, Japanese Unexamined Patent Application Publication No. 11-46126 discloses an example of a ladder filter apparatus. In Japanese Unexamined Patent Application Publication No. 11-46126, series arm resonators and parallel arm resonators are formed of surface acoustic wave resonators. A matching inductor is connected between an input terminal and a surface acoustic wave resonator closest to the input terminal. A matching inductor is connected between an output terminal and a series arm resonator closest to the output terminal.
In a ladder filter apparatus, it is necessary to increase a frequency interval between the resonant frequency of a parallel arm resonator and the anti-resonant frequency of a series arm resonator in order to widen a band. However, when the frequency interval is increased too much, it becomes difficult to perform impedance matching at input/output terminals.
In Japanese Unexamined Patent Application Publication No. 11-46126, matching inductors are connected to input/output terminals. However, due to connection of the matching inductors described above, impedance matching can be performed only in the case where the band width ratio is 4% or less. When the band is widened more than that, insertion loss in the pass band is increased and a ripple in the pass band is increased. Further, there is a problem in that the VSWR characteristics are degraded.